The use of alkyltrialkoxysilanes and their hydrolysates as surface treatments in applications such as clear penetrating water-repellents for masonry is well-established. In other applications, penetration of the substrate must be accompanied by the ability to form a film on the surface substrate. This result can be achieved with relative ease for fixed high modulus “stiff” substrates. In contrast, flexible substrates, such as metallized food packaging, cellulosic substrates including wood and paper, and personal care products with substantivity on hair and skin, are more difficult since the condensation and polymerization products of alkyltrialkoxysilanes (which may be referred to as silsesquioxanes or T resins) are stiff and generally cannot withstand flexure. Even more extreme demands are found for outdoor wood and cosmetic applications, such as hair care formulations, where adhesion permanence and substantivity is defined in terms of rain or wash resistance. This requires formation of water-resistant, flexible, tenacious films with appropriate sensory properties. Compositions useful for this purpose may be generally classified as either water-borne or non-waterborne. The performance, particularly silane penetration of porous substrates and, consequently, resistance to ions (salt), detergents and water transport is generally superior in non-waterborne systems. However, safety and environmental considerations favor the use of water-borne systems.
An intrinsic element of alkylalkoxysilane chemistry generally limits the stability of these materials in aqueous systems. Specifically, the alkoxy group hydrolyzes in water to form an active silanol species. This species can react with the surface hydroxyls of the siliceous substrates, rendering them hydrophobic, or they can react with themselves, forming silsesquioxanes which precipitate out of solution. General considerations of the factors affecting stability of alkoxysilanes in aqueous solution have been reviewed (B. Arkles et al, J. Adhes. Sci & Technol, 6(1), 193 (1992)).
Two approaches are generally employed in current commercial formulation technology. The first approach is to form oligomeric alkylalkoxysilanes and then to emulsify these systems. One example is for mixed alkylalkoxysiloxane oligomers. The oligomeric nature of these materials tends to prevent hydrolysis of the silanes under neutral conditions. On the other hand, the increase in molecular size prohibits deep penetration into submicron pores in siliceous substrates. Another example (see U.S. Pat. No. 4,661,551) are silsesquioxanes generated from mixed hydrolysates of aminoethylaminopropyltrialkoxysilane and alkyltrialkoxysilanes. These high amine-content materials generally discolor substrates and have generally low penetrations since the high temperature hydrolysis results in high molecular weight.
In the second approach (see U.S. Pat. No. 4,648,804), long chain alkyltriethoxysilanes, e.g., octyltriethoxysilane, are stabilized as monomers in a micelle or emulsion form. When the micelle or emulsion breaks down on contact with a masonry substrate, the octyltriethoxysilane is no longer stable and reacts with the substrate. This approach has the same deficiency as the oligomeric silanes in that the long alkyl chain precludes penetration of the silane. Comparative tests of these products under standard NCHR 244 tests indicate that they generally penetrate only 3/16″ compared to at least twice the penetration for systems such as alcohol solutions of isobutyltrimethoxysilane. In a similar approach (see U.S. Pat. No. 6,610,782), a binary emulsion is formed from a combination of an hydrolyzable silane, more specifically an alkyltrialkoxysilane or low molecular weight alkyltrialkoxysilane oligomeric hydrolysate, a low molecular weight silicone, an aminofunctional fluid and an aminofunctional silane in water.
A third approach is exemplified in EP 1 136 494 A3, in which the transesterification of alkyltrialkoxysilanes with polyhydroxyl compounds, usually with the concomitant removal of lower alcohols in aqueous solution, is disclosed.
Still another approach is to stabilize the reactive intermediate, the silanetriol, in aqueous systems. Stabilizing the triol maintains the low molecular weight needed to penetrate the various micro-porous substrates. However, prior work has not been able to provide stable solutions of alkylsilanetriols suitable for commercial water repellents. While U.S. Pat. No. 4,708,743 describes methods for producing solutions of propylsilanetriol that are stable for up to 24 hours, this necessitates on-site mixing, and practical experience indicates that the solutions are usually not stable for more than six hours. Aged solutions have silsesquioxane materials which can stain masonry. Nevertheless, penetration of the propylsilanetriol is intermediate between emulsions of alkoxysilanes and alcohol solutions of alkoxysilanes. Accordingly, stable solutions of alkylsilanetriols for commercial water repellants would be desirable.